The parent applications disclose the breakthrough concept of taking care to control the level of dissolved organic materials (DOM)--which mainly comprise dissolved lignin, hemi-cellulose, cellulose, and other materials extracted from wood by the kraft pulping process--in order to increase pulp strength and bleachability, with decreased consumption of effective alkali, and at a lower H factor. The DOM level is typically kept at a concentration of 100 g/l or less (e.g. about 50 g/l or less) substantially throughout the cook, the hemi-cellulose level typically being kept at 15 g/l or less (e.g. about 10 g/l or less), and the lignin level typically being kept at 50 g/l or less (e.g. about 25 g/l or less). This is done by withdrawing high DOM level liquid from an upright digester and/or feed or circulatory loops associated with the digester (typically a continuous digester), and replacing the high DOM level liquid with low effective DOM level liquid. The term "effective DOM" as used in association with the invention means that portion of the DOM that affects pulp strength, H factor, effective alkali consumption, and/or bleachability. Low effective DOM liquid is typically water, substantially DOM free white liquor, washer filtrate, cold blow filtrate, pressure-heat treated black liquor, and combinations thereof.
According to the present invention, various digester systems and components, and methods of treating comminuted cellulosic fibrous material, are provided which facilitate practical implementation of the DOM level control concepts disclosed in the parent applications. According to the present invention, various advantageous results are obtained, some of them by using unexpected techniques, others by using conventional equipment but in new manners.
In pulping systems which control DOM, adding white liquor at numerous points and levels in a continuous digester, and adding low effective DOM dilution liquids too, means that less liquid is available to transport chips during and after impregnation, that is from the high pressure transfer device to the digester. For example, the amount of alkali fed to the feed system in digester systems controlling DOM may be as low as 40-55% (e.g. less than about 55%). In order to accommodate this, liquid is withdrawn from the circulation line from the top of the digester to the bottom of the impregnation vessel, cooled, and fed to the high pressure inlet line for the high pressure transfer device.
That is, according to one aspect of the present invention, a two vessel continuous digester system for digesting cellulosic fibrous material is provided, including the following elements: An impregnation vessel having a top and a bottom, an inlet for cellulosic fibrous material slurry to be digested at the top of the impregnation vessel, and an outlet for slurry from the bottom of the impregnation vessel, and an outlet for recirculating liquid from the top of the impregnation vessel. A circulatory system for feeding slurry to the top of the impregnation vessel, and receiving recirculating liquid from the top of the impregnation vessel. A continuous digester having a top and a bottom, an inlet for cellulosic fibrous material slurry to be digested at the top of the digester, an outlet for digested pulp at the bottom of the digester, and an outlet for recirculating liquid from the top of the digester. A first conduit extending between the impregnation vessel slurry outlet and the digester slurry inlet, and a second conduit extending between the recirculating liquid outlet at the top of the digester and the bottom of the impregnation vessel. At least one DOM-reducing screen assemblies between the top and bottom of the digester. Means for withdrawing a first portion of the liquid passing through each of the DOM-reducing screen assemblies and passing it to recovery, a second portion of the liquid remaining. Means for adding low effective DOM liquid to each the second portion to provide an augmented second portion; means for adding cooking liquor to at least one of the second portions. Means for recirculating the augmented second portions to the interior of the digester. And, novel according to the invention, means for augmenting the flow of liquid in the circulatory system, comprising means for withdrawing liquid from the second conduit, means for cooling the withdrawn liquid, and means for introducing the cooled withdrawn liquid into the circulatory system.
The circulatory system typically includes a high pressure transfer device and a high pressure pump having an inlet and an outlet, the high pressure pump being connected to the high pressure pump outlet. The means for introducing the cooled withdrawn liquid into the circulatory system comprises means for introducing the cooled withdrawn liquid into a conduit connected to the high pressure pump inlet. The system also preferably further comprises means for adding cooking liquor to the withdrawn liquid, e.g. adding the cooking liquor before the means for cooling the withdrawn liquid. The means for cooling the withdrawn liquid preferably comprises a heat exchanger cooler, or an evaporative cooler.
The digester also preferably includes an extraction screen assembly, and the at least one DOM-reducing screen assemblies comprise a trim screen assembly located adjacent the top of the digester, and a cooking screen assembly disposed between the trim screen assembly and the extraction screen. The digester preferably further includes a wash screen assembly located between the bottom of the digester and the extraction screen assembly, and further comprising means for adding cooking liquor to the wash screen assembly. A blanked recirculation pipe may be disposed in the digester at the extraction screen assembly, so that the extraction screen assembly may be readily modified to comprise a recirculating screen assembly.
Alternatively, the digester may further include a combined recirculating and extraction screen assembly adjacent the top of the digester; and the at least one DOM-reducing screen assemblies preferably includes a first screen assembly disposed below the combined recirculating and extraction screen assembly, and a second screen assembly disposed below the first screen assembly. Also provided may be a wash screen assembly located between the bottom of the digester and the second screen assembly, and means for adding cooking liquor to the wash screen assembly.
According to another aspect of the invention, a method utilizing such apparatus may be practiced. The method comprises the following steps: a) At at least one location between the top and bottom of the digester, withdrawing DOM containing liquid from the interior of the digester, splitting the liquid into first and second flows, and passing the first flow to recovery. (b) Adding low DOM containing liquid to the second flow, including white liquor, and recirculating the second flow back to the interior of the digester; and (c) augmenting the flow liquid in the circulatory system by withdrawing liquid from the second conduit, cooling the withdrawn liquid, and introducing the cooled withdrawn liquid into the circulatory system.
Preferably, the circulatory system includes a high pressure transfer device and a high pressure pump having an inlet and an outlet, the outlet being connected to the high pressure transfer device, in which case step (c) is practiced by introducing the cooled withdrawn liquid into a conduit connected to the high pressure pump inlet, and is preferably further practiced by adding white liquor to the withdrawn liquid (e.g. by adding the cooking liquor to the withdrawn liquid before cooling the withdrawn liquid). Typically, the amount of white liquor to the material prior to the digester is about 55% or less of the total amount of white liquor added. The cooling in step (c) may be practiced by heat exchange or evaporative cooling.
The method also typically includes the further steps of withdrawing and recirculating liquid in a wash loop at the bottom of the digester and adding white liquor to the wash loop, and extracting liquid from the digester and passing it to recovery distinct from the wash loop and the first portion of the withdrawn liquid passed to recovery from step (a). Normally steps (a) and (b) are practiced at a plurality of different levels within the digester.
According to the invention, there may also be provided particular means and a procedure for controlling the level of liquid in a vapor (steam) phase digester. Going against conventional wisdom, which is that it is not beneficial to extract liquor shortly after impregnation during kraft cooking, according to this aspect of the invention, an extraction from near the top of the vessel may be led into operative association with the level tank associated with the feed system for the digester assembly, either directly connected to a conduit leading to the level tank in a single vessel vapor phase system, or to the sluice flow of the high pressure transfer device if a two vessel vapor phase system is utilized.
That is, according to this aspect of the present invention, a vapor phase digester assembly is provided comprising the following components: A continuous digester having a top and a bottom, including a comminuted cellulosic fibrous material inlet at the top of the digester, and a pulp outlet at the bottom of the digester. At least one DOM-reducing screen assembly for withdrawing liquid having a first concentration of DOM, reducing the DOM concentration so that it is at a second concentration, much less than the first concentration, and then reintroducing the second DOM concentration liquid into the interior of the digester. A feed system for feeding a slurry of comminuted cellulosic fibrous material to the top of the digester, including a high pressure transfer device, a chip chute connected to the high pressure transfer device, a high pressure pump connected to the high pressure transfer device, circulatory conduits leading between the high pressure transfer device and the top of the digester, and a level tank operatively connected to the chip chute, and a means for removing liquor from the interior of the digester, this means providing for gross control of the level of liquid in the top of the digester to maintain a vapor phase at the very top of the digester. And, characteristic of the invention, means for effecting fine control of the level of liquid in the top of the digester to maintain a vapor phase at the very top of the digester, the means for effecting fine control comprising: a level control screen located below the liquid level adjacent the top of the vessel, but above other screens; a conduit leading from the level control screen and splitting into first and second branches, the first branch recirculating liquid to the interior of the digester, and the second branch operatively connected to the level tank to facilitate fine control of the liquid level in the digester.
The assembly typically also includes means for cooling the liquid in the second branch between the digester and the level control tank, and a third branch branching off of the second branch, the third branch leading to recovery. Valve means may also be provided for controlling the relative amount of liquid flowing in the second and third branches.
A two vessel system may also be utilized, in which an impregnation vessel disposed between the high pressure transfer device and the digester in the circulatory conduits, and including a sluice conduit. In that case, the second branch is connected to the sluice conduit.
The invention also relates to a method of treating comminuted cellulosic fibrous material using this system, including a continuous digester having a top and a bottom, including a comminuted cellulosic fibrous material inlet at the top of the digester, and a pulp outlet at the bottom of the digester. The method comprises the following steps: At at least one location in the digester, effecting reduction of the DOM of the liquid in the digester by withdrawing liquid having a first concentration of DOM, reducing the DOM concentration so that it is at a second concentration, much less than the first concentration, and then reintroducing the second DOM concentration liquid into the interior of the digester. Feeding a slurry of comminuted cellulosic fibrous material to the top of the digester, including using a high pressure transfer device, a chip chute connected to the high pressure transfer device, a high pressure pump connected to the high pressure transfer device, circulatory conduits leading between the high pressure transfer device and the top of the digester, and a level tank operatively connected to the chip chute. Effecting gross level control of the level of liquid in the top of the digester to maintain a vapor phase at the very top of the digester; and effecting fine control of the level of liquid in the top of the digester to maintain a vapor phase at the very top of the digester, by withdrawing liquid from below the vapor phase at the top of the digester, but near the top of the vessel, splitting the withdrawn liquid into first and second branches, recirculating liquid in the first branch to the interior of the digester, and circulating liquid in the second branch to the level tank to facilitate fine control of the liquid level in the digester.
The method may also comprise the further step of cooling the liquid in the second branch between the digester and the level control tank. Where there is a third branch branching off of the second branch, here may be the further steps of leading the third branch to recovery, and controlling the relative amount of liquid flowing in the second and third branches.
According to another aspect of the invention, an extraction is taken from the outlet of the feed system make-up liquor pump. This allows the removal of DOM-rich liquor from the feed system (which is particularly advantageous since in means the DOM level at the beginning of the cook is less), while retaining fines or pin chips that may be present. The in-line drainer may be used with a single vessel or two vessel system, and with hydraulic or vapor phase digesters.
Also according to the present invention, it is important to design the extraction/dilution screens so as to minimize mixing between liquors of different DOM concentrations, while still also accommodating the direction of flow of chips and liquids in the vicinity of the screens. This is accomplished by providing first and second screens inside the digester, and at least one liquid re-introduction conduit for reintroducing liquid withdrawn through the extraction/dilution screens adjacent the volume of withdrawal, and having a liquid introducing opening therein. The first and second screens are vertically spaced from each other, the closest portions of the first and second screens spaced from each other a first distance. The liquid introducing opening in the re-introduction conduit is closest to the second screen and spaced from the closest portion of the second screen a second distance. The first distance is 0-10 feet (preferably about 4-6 feet), and the second distance is 0-10 feet (preferably about 3-6 feet). In countercurrent flow situations, the second screen is vertically above the first screen and the reintroduction conduit opening is above the second screen. In co-current flow situations, the second screen is vertically below the first screen and the reintroduction conduit opening is below the second screen.
Typically, one or both of the screens is connected to a flash tank through the withdrawal conduit, and at least one of the screens is connected to the reintroduction conduit. The digester also typically further comprising at least one screen assembly for withdrawing DOM rich liquid, splitting the liquid into first and second flows, passing the first flow to recovery; and adding low DOM liquid to the second flow, and reintroducing the second flow to the digester to reduce the concentration of DOM in the digester.
It is the primary object of the present invention to provide various systems and methods for enhancing the versatility and practicality of kraft pulping taking into account DOM, typically holding the DOM concentration below 100 g/l throughout the entire cook. This and other aspects of the invention will become clear from an inspection f the detailed description of the invention, and from the appended claims.